The present invention relates to an apparatus for determining the properties of a medium in the form of a fluid or a soft material, in particular a highly viscous, dough-like or pasty medium.
A generic apparatus is described in WO 2008/034878 A2. It includes an acoustic waveguide which is formed by at least two opposing guide elements in the form of two plates and encloses an interior space which is filled with the medium to be measured. For filling the interior space and for carrying out measurements, in order to determine the properties of the medium, the waveguide for example is immersed into the medium or the medium is filled into the interior space of the waveguide or guided through the same.
Via a transmitter, acoustic surface waves are generated in the waveguide, wherein at least a part of the energy of the acoustic surface waves is coupled into the medium, so that a part of the energy of the acoustic surface waves is converted into volumetric sound waves of the medium.
Due to the reciprocity of this coupling process between a plate of the waveguide sensor and the medium, a part of the volumetric sound waves propagating in the medium is again coupled back into one or both of the plates of the waveguide, so that acoustic surface waves are generated therein. By measuring and evaluating certain characteristics of these acoustic surface waves, such as their velocity or their amplitudes, chemical and/or physical properties of the medium can be determined.
In principle, the volumetric sound waves are coupled out into the medium under an angle δ relative to a vertical reference line of a plate of the waveguide:δ=arcsin(cM/cS).
Herein, cM is the sound velocity of the volumetric sound waves inside the medium and cS is the sound velocity of the acoustic surface waves propagating along a plate of the acoustic waveguide.
In the apparatus described in WO 2008/034878 A2, the (carrier) plates or guide elements of the waveguide sensor are arranged separate from each other at a housing carrying the same. The plates as guide elements of the waveguide each form a sealing cover for a cavity opposite the interior space to be filled with the medium. In this cavity, a transmitter and/or a receiver for generating or receiving the acoustic surface waves are arranged and possibly are directly attached to an outer surface of a guide element facing the cavity.
This form of construction, however, is relatively expensive to manufacture, as it must be ensured that the guide elements guiding the acoustic surface waves seal the cavity associated to them against the entry of the medium. In addition, the design of the separately mounted guide elements with regard to the occurring mechanical loads and for a desired long-term stability of the waveguide formed therewith only can be ensured in a relatively expensive and cost-intensive way.